System and method for steering a multi-wheel drive vehicle

ABSTRACT

A system and method of controlling a multi-wheel drive vehicle is provided. The invention is preferably applicable to the steering of such a vehicle and determines the individual velocities for each wheel drive. In this regard, the invention includes two general steps. The first step includes determining the distance of each wheel drive and a vehicle velocity reference point from a turning reference point. The second step includes ratioing each wheel drive&#39;s distance from the turning reference point with the vehicle velocity reference point&#39;s distance from the turning reference point. The ratios are then applied to a vehicle velocity associated with the vehicle velocity reference point to determine the velocity of each respective wheel drive. Once determined, the velocities are output to each wheel drive.

FIELD OF THE INVENTION

[0001] The invention relates generally to multi-wheel drive vehiclesand, more particularly, to the steering of mult-wheel drive vehiclessuch as, for example, wheelchairs and the like.

BACKGROUND OF THE INVENTION

[0002] Generally, a multi-wheel drive vehicle is any vehicle that hasmore than one wheel that is used to power or drive the vehicle. Examplesof such vehicles include two and four wheel drive wheelchairs. Drivingsuch vehicles in straight lines does not pose significant controlissues. However, this is generally not the case when driving suchvehicles into turns. More specifically, to drive a multi-wheel drivevehicle into a turn requires that each of the wheel drives havediffering velocities in order to achieve the turn.

[0003] If these differing velocities are not properly realized in thewheel drives, several undesirable consequences result. For example, ifone or more of the wheel drives has a velocity below that required tomake the turn at a particular vehicle speed, those affected wheel driveswill at least partially “drag” along the riding surface. This isundesirable for several reasons. First, it creates wear on the wheelcomponent of the wheel drive. Second, it creates wear on the ridingsurface. If the riding surface is, for example, a carpet, then suchcarpet may be damaged by such dragging action. If one or more of thewheel drives has a velocity above that required to make the turn, thevehicle will not effectively make the desired turn because the vehiclesuffers from understeer. Additionally, such non-optimal control of thewheel drives leads to higher energy consumption by the vehicle, whichsignificantly reduces the vehicle's range. Hence, it desirable toprovide a system and method for controlling a multi-wheel drive vehiclethat does not suffer these drawbacks.

SUMMARY OF THE INVENTION

[0004] According to one embodiment of the present invention, a method ofcontrolling a multi-wheel drive vehicle is provided. The method ispreferably applicable to the steering of such a vehicle and determinesthe individual velocities for each wheel drive. In this regard, themethod includes two general steps. The first step includes determiningthe distance of each wheel drive and a vehicle velocity reference pointfrom a turning reference point. The second step includes ratioing twocomponents: the distance from the turning reference point to each wheeldrive and the distance from the turning reference point to the vehiclevelocity reference point. The ratios are then applied to a vehiclevelocity that is associated with the vehicle velocity reference point,to determine the velocity of each respective wheel drive. Oncedetermined, the velocities are output to each wheel drive.

[0005] According to another embodiment of the present invention, amethod of controlling a multi-wheel drive vehicle includes, for example,the step of determining a turning reference, vehicle velocity, andreference distance. The reference distance is the distance between theturning reference and a known reference position relative to thevehicle. The method further includes, for example, determining a wheeldrive distance and velocity for each wheel drive. The wheel drivedistance is the distance of each wheel drive from the turning reference.The velocity for a wheel drive is determined from the wheel drivedistance, reference distance and vehicle velocity. Once the velocity foreach wheel drive has been determined, it is outputted to each wheeldrive.

[0006] According to another embodiment of the present invention a systemfor controlling a multi-wheel drive vehicle is provided. The systemincludes, for example, an input device, a controller in circuitcommunication with the input device, at least two wheel drives incircuit communication with the controller, and logic for determining theindividual velocities for the at least two wheel drives.

[0007] Hence, the present invention is particularly useful for anyvehicle having two or more wheel drives. The present invention is alsoapplicable to any vehicle that allows for individual wheel drivecontrol. Such vehicles include electrically driven vehicles andcombustion engine driven vehicles. The present invention is alsoparticularly useful in that it can replace conventional transmissionsincluding differential gearboxes, four wheel coupling transmissions, andspecial “visco” clutches, just to name a few. It should also be notedthat the present invention can also be used with such conventionaltransmissions that allow for individual wheel drive control.

[0008] Therefore, it is an advantage of the present invention to providea method of steering a multi-wheel drive vehicle by determining thevelocity of each individual wheel drive.

[0009] It is another advantage of the present invention to provide asystem for steering a multi-wheel drive vehicle that includes a meansfor determining the individual velocities of each wheel drive.

[0010] It is yet another advantage of the present invention to provide asurface tolerant drive system that includes a means for driving avehicle when the wheel drives are on differing surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the accompanying drawings which are incorporated in andconstitute a part of the specification, embodiments of the invention areillustrated, which, together with a general description of the inventiongiven above, and the detailed description given below, serve to examplethe principles of this invention.

[0012]FIG. 1 is a general block diagram of a control system 100 of thepresent invention.

[0013]FIG. 2 is a block diagram illustrating the spatial relationshipsof the wheel drives and reference points of one exemplary embodiment ofthe present invention.

[0014]FIG. 3 is a block diagram further illustrating the spatialrelationships of the wheel drives and reference points of one exemplaryembodiment of the present invention.

[0015]FIG. 4 is a flowchart generally illustrating the logic stepsexecuted by a first embodiment of the present invention to determine theindividual wheel drive velocities.

[0016]FIG. 5 is a flowchart generally illustrating the logic stepsexecuted by a second embodiment of the present invention to determinethe individual wheel drive velocities.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENT

[0017] Illustrated in, FIG. 1 is a block diagram of a system 100 of thepresent invention. System 100 has a controller 102, logic 104, inputdevice 106, and wheel drives 108, 110, 112, and 114. Controller 102 isany computer-based controller suitable for controlling a vehicle. Inthis regard, controller 102 generally has a processor, memory, andinput/output components (not shown). Controller 102 can also haveelectric motor drive circuitry associated therewith (not shown).

[0018] Input device 106 is in circuit communication with controller 102and provides input signals thereto. More specifically, input device 106is preferably a user manipulable device such as a joystick or othersimilar device. In this regard, input device 106 at least providesvehicle velocity and turning vector information. Generally, such vectorinformation is in the form of magnitude and direction information. Inits simplest form, input device 106 outputs “x” and “y” Cartesiancoordinate information wherein the “x” coordinate relates to turningvector information and the “y” coordinate relates to velocity vectorinformation.

[0019] Wheel drives 108, 110, 112, and 114 are also in circuitcommunication with controller 102. Each wheel drive preferably as awheel component and a drive component. The drive component is preferablyresponsible for driving the wheel component and can be in the form ofany one of a number of embodiments. For example, one embodiment of adrive component is a gearless, or transmissionless, brushless electricmotor. Another embodiment of a drive component is a brushed electricmotor and a gearbox, or transmission, communicating with the wheelcomponent. Each wheel drive may also have electric motor drive circuitryassociated therewith that receives drive control signals from controller102 and correspondingly translates these control signals to properlydrive the wheel drive. Other embodiments include combustion engines withelectronic or hydraulic transmissions. For example, a single combustionengine may be coupled with a transmission system that allows each drivewheel to be driven at a different velocity. Further variations of theseembodiments are also intended to fall within the scope of the presentinvention. Hence, the present invention is applicable to and includesnumerous embodiments of wheel drives.

[0020] Also in circuit communication with controller 102 is a steeringservo-mechanism 116. The steering servo mechanism 116 is in physicalcommunication with wheel drives 112 and 114 and physically drives theangular position of such wheel drives. In this regard, steering servomechanism 116 has associated therewith a position resolver orpotentiometer (not shown), which outputs the steering servo mechanism's116 angular position β. (See FIG. 2). One example of a common steeringservo-mechanism is a rack-and-pinion mechanism. Other types of steeringservo-mechanisms also exist that can be employed.

[0021] Logic 104 of the present invention determines the velocity ofeach wheel drive 108, 110, 112, and 114. As will be described inconnection with FIGS. 2 through 5, logic 104 employs vehicle velocityand turning information to appropriately determine the required wheeldrive velocities to ensure when driving into a turn that the turn ispreferably taken without any of the wheel drives dragging on the ridingsurface.

[0022] In the preferred embodiment, system 100 is applied to a vehiclesuch as, for example, a four-wheel drive wheelchair. However, system 100is applicable to other multi-wheel drive vehicles such as scooters, golfcarts, and all-terrain vehicles just to name a few examples. Generally,any vehicle having more then one wheel drive can benefit from thepresent invention.

[0023] Referring now to FIG. 2, the present discussion will now focus onthe general logic of the present invention as it applies to, forexample, a four-wheel drive vehicle. More specifically, FIG. 2illustrates several reference points and dimensions that are employed bylogic 104 with reference to the location of wheel drives 108, 110, 112,and 114. In this regard, the wheel base positions or dimensions aredefined as H_(BL), and W_(BL) for the left rear wheel drive 112 andH_(BR) and W_(BR) for right rear wheel drive 114. The wheel basepositions or dimensions for the front left and right wheel drives 108and 110 are defined by W_(FL) and W_(FR). The above-defined wheel drivebase dimensions H and W values denote vertical and lateral distances,respectively, from a reference point “O,” which is the center or originof the vehicle's coordinate system. Additionally, these values are allstored in the memory of controller 102 for use by logic 104, as will beexplained below.

[0024] A turning reference point “P” exists that is a turning referencedistance “a” from point “O”. Turning reference point “P” defines a pointabout which the vehicle desires to turn. A third point “R” existslaterally between wheel drives 112 and 114 and spatially defines thevehicle's velocity reference point. The steering servo-mechanism 116 ismechanically coupled to the rear wheels. The steering servo-mechanism116 is typically a geared motor, which drives a lever and a positionfeedback potentiometer. The feedback potentiometer defines an angularposition in terms of an angle β, as shown. The angle β is used todetermine the turning reference distance a between turning referencepoint “P” and point “O”, as shown in FIG. 2. This determination is madevia Equation (1):

a=H _(R)×tan β  Eq. (1)

[0025] where β is the angle formed between the “y” axis running throughpoint “O” (positive direction shown) and the vehicle's velocity vectorV_(R), and H_(R) is the vertical distance between point “O” and thevelocity reference point “R”. For example, if H_(R)=1 then when angle βis 45 degrees, the turning reference distance a between turningreference point “P” and point “O” is equal to one. In this example,turning reference distance of a=1 represents a turning radius of 1 yard,meter, foot, or other units as desired. The location of turningreference point “P” relative to point “O” is determined via Equation(2):

P=−H _(R)×cot(β)=H _(R)×cot(−β)   Eq. (2)

[0026] Having defined the turning reference distance a and the wheelbase positions, reference is now made to FIG. 3. FIG. 3 illustratesdistances S from turning reference point “P” to each wheel drive and thevelocities V of each wheel drive. In this regard, logic 104 employseither of two approaches to determine the individual wheel drivevelocities. The first approach utilizes a two step analysis wherein thefirst step is to determine, from turning reference point “P”, thedistance of each of the wheel drives and the vehicle's velocityreference point “R”. Once these distances are known, the individualwheel drive velocities can be determined based on the known velocity atthe vehicle's velocity reference point “R” and the wheel drive'sdistance from turning reference point “P”. The second approach utilizesthe steering servo-mechanism's 116 angular positions β and the vehicle'swheel drive base dimensions.

[0027] Referring now to FIG. 3, the present discussion will now focus onthe first approach described above. In this regard, distance S_(FL) isthe distance between wheel drive 108 and turning reference point “P.”Similarly distances S_(FR), S_(BL), and S_(BR) represent the distancesbetween wheel drives 110, 112, and 114, respectively, and turningreference point “P.” Distance S_(R) represents the distance betweenvehicle's velocity reference point “R” and turning reference point “P.”So defined, these distances can be determined according to Equations(3)-(7): $\begin{matrix}{S_{FL} = {a - \frac{w}{2}}} & \text{Eq.~~(3)} \\{S_{FR} = {a + \frac{w}{2}}} & \text{Eq.~~(4)}\end{matrix}$

S _(BL)={square root}{square root over ((a−W _(BL))² +H _(BL) ²)}  Eq.(5)

S _(BR)={square root}{square root over ((a+W _(BR))² +H _(BR) ²)}  Eq.(6)

S _(R) ={square root}{square root over (a²+H_(R) ²)}  Eq. (7)

[0028] Once all of the wheel drive distances S from turning referencepoint “P” have been determined, the vehicle's velocity V_(R) and theindividual velocities V_(BL), V_(BR), V_(FL), and V_(FR), for wheeldrives, 112, 114, 108, 110, respectively, can be determined. In thisregard, velocities V_(R), and V_(BL) and V_(BR) for wheel drives 112 and114 can be determined from Equations (8)-(10):

V _(R)=ƒ(x,y)   Eq. (8)

[0029] $\begin{matrix}{V_{BL} = {\frac{S_{BL}}{S_{R}} \times V_{R}}} & \text{Eq.~~(9)} \\{V_{BR} = {\frac{S_{BR}}{S_{R}} \times V_{R}}} & \text{Eq.~~(10)}\end{matrix}$

[0030] In Equation (8), the vehicle's velocity V_(R) is a functionƒ(x,y) of the output signals of input device 106. In this regard, The“x” coordinate provides data for determining a turning vector'smagnitude and direction and the “y” coordinate provides data relevant toboth the vehicle's velocity vector magnitude and direction and theturning vector magnitude and direction. Typically, the function ƒ(x,y)also provides for, among other things, limitations on the vehicle's topvelocity, rate of acceleration, and/or rate of deceleration, for safetyand proper operation of the vehicle. The exact parameters of functionƒ(x,y) are generally determined based on a particular wheelchair'sdesign. Also, the vehicle's turning vector information is typically afunction g(x,y) of both the “x” and “y” coordinate data. In this regard,the “y” coordinate (or velocity data) is typically used to reduce themagnitude of the turning vector at higher vehicle velocities forpurposes of safety (e.g., to prevent the vehicle from flipping orrolling over.) Once again, the exact parameters of g(x,y) are determinedbased on a particular wheelchair's design. Additionally, the velocityfunction ƒ(x,y) and turning function g(x,y) may be implemented using thepolar output information (r,θ). Hence, the velocity function would be inthe form of ƒ(r,θ) and the turning function would be in the form ofg(r,θ), where the parameters of each function are determined based on awheelchair's particular design. Moreover, the turning function g(x,y) org(r,θ) can itself be modified by a function m(g(x,y)) to account for thedifference between the turning function's angular output result and theservo-mechanism's actual angular position. Such differences arise, forexample, because the servo-mechanism requires a finite time betweenchanges in angular positioning and because the actual servo-mechanismangular position may be slightly different from that determined via theoutput of input device 106. Once the reference velocity V_(R) isdetermined, it is stored in the memory of controller 102 for use indetermining the individual wheel drive velocities.

[0031] The equations for determining velocities V_(FL), and V_(FR) forwheel drives 108 and 110, respectively, are dependent on angle β and canbe shown to be defined by Equations (11)-(14): $\begin{matrix}\begin{matrix}{V_{FL} = {\frac{S_{FL}}{S_{R}} \times V_{R}}} & {{\text{for}\quad 0} \geq \beta \geq {- \pi}}\end{matrix} & \text{Eq.~~(11)} \\\begin{matrix}{V_{FR} = {\frac{S_{FR}}{S_{R}} \times V_{R}}} & {{\text{for}\quad 0} \geq \beta \geq {- \pi}}\end{matrix} & \text{Eq.~~(12)} \\\begin{matrix}{V_{FL} = {\frac{- S_{FL}}{S_{R}} \times V_{R}}} & {{\text{for}\quad 0} \leq \beta \leq \pi}\end{matrix} & \text{Eq.~~(13)} \\\begin{matrix}{V_{FR} = {\frac{- S_{FR}}{S_{R}} \times V_{R}}} & {{\text{for}\quad 0} \leq \beta \leq \pi}\end{matrix} & \text{Eq.~~(14)}\end{matrix}$

[0032] Generally, Equations (9)-(14) determine each wheel drive'svelocity V by ratioing each wheel drive distance S with the vehicle'svelocity reference point distance S_(R) and then applying that ratio tothe vehicle velocity V_(R) designated at the vehicle velocity referencepoint “R” . Hence, by determining the individual wheel drive distances,the individual wheel drive velocities can be determined therefrom.

[0033] Still referring to FIG. 3, the second approach described aboveutilizes the steering servo-mechanism's 116 angular position β and thevehicle's wheel drive base dimensions to determine each individual wheeldrive velocity. In this regard, wheel drive velocities V_(BL), V_(BR),V_(FL), and V_(FR) can be defined by the following Equations (15)-(18)derived from FIG. 3: $\begin{matrix}{V_{BL} = {V_{R} \times \frac{\sqrt{\left( \left\lbrack {{H_{R} \times {\cos \left( {- \beta} \right)}} - {\frac{W_{B}}{2} \times {\sin \left( {- \beta} \right)}}} \right\rbrack \right)^{2} + \left( \left\lbrack {H_{B} \times \sin \quad \left( {- \beta} \right)} \right\rbrack \right)^{2}}}{H_{R}}}} & \text{Eq.~~(15)} \\{V_{BR} = {V_{R} \times \frac{\sqrt{\left( \left\lbrack {{H_{R} \times {\cos \left( {- \beta} \right)}} + {\frac{W_{B}}{2} \times {\sin \left( {- \beta} \right)}}} \right\rbrack \right)^{2} + \left( \left\lbrack {H_{B} \times \sin \quad \left( {- \beta} \right)} \right\rbrack \right)^{2}}}{H_{R}}}} & \text{Eq.~~(16)} \\{V_{FL} = {V_{R} \times \left\lbrack {{\cos \left( {- \beta} \right)} - {\frac{W_{F}}{2 \times H_{R}} \times {\sin \left( {- \beta} \right)}}} \right\rbrack}} & \text{Eq.~~(17)} \\{V_{FR} = {V_{R} \times \left\lbrack {{\cos \left( {- \beta} \right)} + {\frac{W_{F}}{2 \times H_{R}} \times {\sin \left( {- \beta} \right)}}} \right\rbrack}} & \text{Eq.~~(18)}\end{matrix}$

[0034] In Equations (15)-(18), wheel drive base dimensionsW_(B)=W_(BL)+W_(BR) and W_(F)=W_(FL)+W_(FR) and reference dimensionH_(B)=H_(BL)=H_(BR) and V_(R) is determined via Eq. (8). (See, forexample, FIG. 3 showing the aforementioned dimensions and angle).Additional drive wheels that are connected to a steering servo-mechanismwould follow the general form of Equations (15) and (16) such as, forexample, in the case of a six-wheel drive vehicle that has foursteerable wheel drives. Equations (15)-(18) are similar to Equations(9)-(14) except that Equations (15)-(18) incorporate the steeringservo-mechanism's 116 angular position β and, therefore, simplify thedetermination of V_(FL) and V_(FR). Hence, the individual wheel drivevelocities can be determined based on the steering servo-mechanism's 116angular position β and the wheel base dimensions of the vehicle.

[0035] A third embodiment of the present invention provides for thedetermination of the steering angle for individually steerable wheeldrives. It should also be noted that both previous embodiments of theinvention can also be derived from the following general approach. Inthis regard, the positions of reference points “P” and “R” and wheeldrives 108, 110, 112, and 114 can be determined via Equation (2) andphysical measurement of the vehicle's dimensions, respectively. Thesepositions can then be generally represented as follows:

P=(X_(P),0)

Wheel drive 108=(X_(FL),0)

[0036] Wheel drive 110=(X_(FR),0)

R=(X_(R),Y_(R))

Wheel drive 112=(X_(BL),Y_(BL))

Wheel drive 114=(X_(BR),Y_(BR))

[0037] The steering angle for wheel drive 112 can then be determined viaEquations (19)-(22): $\begin{matrix}{V_{{BL}_{Y}} = {{\left( {X_{R} - X_{BL}} \right) \times \frac{V_{RX}}{Y_{R}}} + V_{RY}}} & \text{Eq.~~(19)} \\{V_{{BL}_{X}} = {Y_{BL} \times \frac{V_{RX}}{Y_{R}}}} & \text{Eq.~~(20)} \\\begin{matrix}{{\angle \quad \overset{\rightarrow}{V_{BL}}} = {\arctan \left( \frac{V_{{BL}_{y}}}{V_{{BL}_{x}}} \right)}} & {{\text{for}\quad V_{{BL}_{x}}} \geq 0}\end{matrix} & \text{Eq.~~(21)} \\\begin{matrix}{{\angle \quad \overset{\rightarrow}{V_{BL}}} = {{{arc}\quad {\tan \left( \frac{V_{{BL}_{y}}}{V_{{BL}_{x}}} \right)}} + \pi}} & {{\text{for}\quad V_{{BL}_{x}}} < 0}\end{matrix} & \text{Eq.~~(22)}\end{matrix}$

[0038] wherein V_(RX)=V_(R) sin β and V_(RY)=V_(R) cos β. Equations (19)and (20) determine the “x” and “y” components of velocity V_(BL) forwheel drive 112. Equations (21) and (22) determine the angle based onthe sign of VBLX. The velocity VBL can be determined via Equation (23):

V _(BL)={square root}{square root over ((V _(BLx))²+(V_(BLy))²)}  Eq.(23)

[0039] The velocities and steering angles for wheel drives 108, 110, and114 can be similarly determined. Once the velocities and steering angleshave been determined for all of the wheel drives, they can be output toeach wheel drive to effect movement of the vehicle.

[0040] Illustrated in FIG. 4 is a flowchart 400 representation of theabove-described logic employing Equations (1) and (8)-(14). The logicstarts in step 402 where the logic reads the angular position β ofsteering servo-mechanism 1116. The logic next advances to step 404 wherethe reference distance a, individual wheel drive distances S, andvehicle velocity reference point distance S_(R) are determined viaequations (1) and (3)-(7), respectively. After step 404, the logic,advances to step 406 where the velocities for each wheel drive aredetermined based on each wheel drive's distance S, the vehicle velocityreference point distance S_(R), the overall vehicle velocity V_(R), andangle β. (See, for example, FIG. 3 and Equations (8)-(14)). After all ofthe wheel drive velocities have been determined, they are output in step408 to the wheel drives. After step 408, the logic loops back to step402 and the logic repeats. It should also be noted that an additionalstep can be included for individually steerable wheel-drives thatincludes outputting the steering angle for each wheel drive to theindividual wheel drive for effecting movement of the vehicle.

[0041]FIG. 5 illustrates a flowchart 500 representation of theabove-described logic employing Equations (8) and (15)-(18). In thisregard, the logic starts in step 502 where the angular position β ofsteering servo-mechanism 116 is read. The logic next advances to step504 where the reference velocity and the velocities for each wheel driveare determined via equations (8) and (15)-(18). In step 506, the logicoutputs the individual velocities to the appropriate wheel drives. Afterstep 506, the logic loops back to step 502 and the logic repeats. Onceagain, it should also be noted that an additional step can be includedfor individually steerable wheel-drives that includes outputting thesteering angle for each wheel drive to the individual wheel drive foreffecting movement of the vehicle.

[0042] In summary, by determining the individual wheel drive velocitiesfor a mult-wheel drive vehicle, accurate turns may be taken by thevehicle that do not cause oversteer, understeer, or any of the wheeldrives to drag along the riding surface. The velocities can bedetermined by either of two approaches. The first approach generallyutilizes a two step analysis. The first step determines the distance ofeach wheel drive from a turning reference point. Once the distances havebeen determined, the velocities are determined in a second step byratioing the wheel drive distance from the turning reference point andthe vehicle's velocity reference point distance from the turning pointand applying that ratio to the vehicle velocity. The second approachutilizes the steering servo-mechanism's angular position and thevehicle's wheel drive base dimensions, along with the vehicle velocity,to determine the individual wheel drive velocities. Once all of theindividual wheel drive velocities have been determined, they are appliedto the wheel drives. Moreover, for individually steerable wheel drives,the steering angle can be determined through the wheel drive's velocity“x” component and “y” component.

[0043] In this manner, each wheel drive is given its own velocity thatis associated with the velocity and turning vector information presentedby input device 106 so as to eliminate oversteer, understeer, and wheeldrive drag during turning. Additionally, the vehicle is provided with asurface tolerant drive system that provides independent wheel drivevelocities. For example, if a first wheel drive is on a normal roadsurface and a second wheel drive is on a complicated surface such as,for example, an ice surface, the vehicle will continue to be driven byvirtue of the drive provided by the first wheel drive.

[0044] While the present invention has been illustrated by thedescription of embodiments thereof, and while the embodiments have beendescribed in considerable detail, it is not the intention of theapplicant to restrict or in any way limit the scope of the appendedclaims to such detail. Additional advantages and modifications willreadily appear to those skilled in the art. For example, the “x” and “y”coordinates of each wheel drive can also be used to determine each wheeldrive's respective distance from the turning reference. The velocity ofany given wheel drive may be allowed to vary or deviate within a smallrange to account for vertical wheel movements due to, for example,obstacles, or small misalignments in the vehicles geometry or othernon-idealities. Therefore, the invention, in its broader aspects, is notlimited to the specific details, the representative apparatus, andillustrative examples shown and described. Accordingly, departures canbe made from such details without departing from the spirit or scope ofthe applicant's general inventive concept.

I claim:
 1. A method of controlling a multi-wheel drive vehiclecomprising the steps of: (a) determining a turning reference and avehicle velocity; (b) determining a reference distance from the turningreference; (c) determining a wheel drive distance from the turningreference for each wheel drive of the multi-wheel drive vehicle; (d)determining a velocity for each wheel drive based on the vehiclevelocity, wheel drive distance, and reference distance; and (e)outputting the determined velocity for each wheel drive to each wheeldrive.
 2. The method of claim 1 wherein step (a) comprises reading theposition output of a user manipulable control device.
 3. The method ofclaim 1 wherein step (a) comprises reading'the angular position of asteering servo-mechanism.
 4. The method of claim 2 wherein step ofreading the position output of a user manipulable control devicecomprises the step of relating Cartesian output data to the tangent ofan angle formed by the Cartesian output data.
 5. The method of claim 1wherein step (a) comprises determining the turning reference based onthe following relationship: a=H _(R)×tan β where a is the turningreference, H_(R) is the distance from an origin of the vehicle'scoordinate system to a vehicle velocity reference point, and β is anangle associated with the vehicle's steering servo-mechanism.
 6. Themethod of claim 1 wherein step (b)-comprises determining the referencedistance based on the following relationship: S _(R) ={squareroot}{square root over (a²+H²)} where S_(R) is the reference distance, ais the turning reference, and H is a wheel base dimension of thevehicle.
 7. The method of claim 1 wherein step (d) comprises determiningthe velocity for each wheel drive based on the following relationship:$V = {\frac{S}{S_{R}} \times V_{R}}$

where V is the velocity for the wheel drive, S is the wheel drivedistance from the turning reference, S_(R) is the reference distance,and V_(R) is the vehicle velocity.
 8. The method of claim 1 furthercomprising the step of determining a steering angle for at least onewheel drive.
 9. The method of claim 9 further comprising the step ofoutputting the determined steering angle to the at least one drive. 10.A system for controlling a multi-wheel drive vehicle comprising thesteps of: (a) an input device; (b) a controller in circuit communicationwith the input device; (c) at least two wheel drives in circuitcommunication with the controller; and (d) logic for: (1) determining aturning reference and a vehicle velocity from the input device; (2)determining a reference distance from the turning reference; (3)determining a wheel drive distance from the turning reference for eachwheel drive of the multi-wheel drive vehicle; (4) determining a velocityfor each wheel drive based on the vehicle velocity, wheel drivedistance, and reference distance; and (5) outputting the determinedvelocity for each wheel drive to each wheel drive.
 11. The system ofclaim 10 wherein the input device comprises a user manipulable inputdevice.
 12. The method of claim 10 wherein the input device comprises asteering servo-mechanism.
 13. The system of claim 11 wherein the usermanipulable input device comprises a joystick input device.
 14. Thesystem of claim 10 wherein the logic determining a turning reference anda vehicle velocity from the input device comprises logic for determiningthe turning reference based on the following relationship: a=H _(R)×tanβ where a is the turning reference, H_(R) is the distance from an originof the vehicle's coordinate system to a vehicle velocity referencepoint, and β is an angle associated with the vehicle's steeringservo-mechanism.
 15. The system of claim 10 wherein the logic fordetermining a reference distance from the turning reference compriseslogic for determining the reference distance based on the followingrelationship: S _(R) ={square root}{square root over (a²+H²)} whereS_(R) is a reference distance, a is the turning reference, and H is awheel base dimension of the vehicle.
 16. The system of claim 10 whereinthe logic for determining a velocity for each wheel drive based on thevehicle velocity, wheel drive distance, and reference distance compriseslogic for determining the velocity for each wheel drive based on thefollowing relationship: $V = {\frac{S}{S_{R}} \times V_{R}}$

where V is the velocity for the wheel drive, S is the wheel drivedistance from the turning reference, S_(R) is the reference distance,and V_(R) is the vehicle velocity.
 17. The method of claim 10 furthercomprising logic for determining a steering angle for at least one wheeldrive.
 18. The method of claim 17 further comprising logic foroutputting the determined steering angle to the at least one drive. 19.A system for controlling a multi-wheel drive vehicle comprising thesteps of: (a) means for inputting at least one control signal; (b) acontroller means in circuit communication with the means for inputting aplurality of control signals; (c) at least two wheel drive means incircuit communication with the controller means; (d) means fordetermining a turning reference and a vehicle velocity from the inputdevice; (e) means for determining a reference distance from the turningreference; (f) means for determining a wheel drive distance from theturning reference for each wheel drive of the multi-wheel drive vehicle;(g) means for determining a velocity for each wheel drive based on thevehicle velocity, wheel drive distance, and reference distance; and (h)means for outputting the determined velocity for each wheel drive toeach wheel drive.
 20. The system of claim 19 wherein the means forinputting at least one control signal comprises a user manipulablemeans.
 21. The system of claim 20 wherein the user manipulable meanscomprises a joystick device.
 22. The method of, claim 19 wherein themeans for inputting at one control signal comprises a steeringservo-mechanism.
 23. The system of claim 19 wherein the means fordetermining a turning reference and a vehicle velocity from the meansfor inputting comprises means for determining the turning referencebased on the following relationship: a=H _(R)×tan β where a is theturning reference, H_(R) is the distance from an origin of the vehicle'scoordinate system to a vehicle velocity reference point, and β is anangle associated with the vehicle's steering servo-mechanism.
 24. Thesystem of claim 19 wherein the means for determining a referencedistance from the turning reference comprises means for determining thereference distance based on the following relationship: S _(R) ={squareroot}{square root over (a²+H²)} where S_(R) is the reference distance, ais the turning reference, and H is a wheel base dimension of thevehicle.
 25. The system of claim 19 wherein the means for determining avelocity for each wheel drive based on the vehicle velocity, wheel drivedistance, and reference distance comprises means for determining thevelocity for each wheel drive based on the following relationship:$V = {\frac{S}{S_{R}} \times V_{R}}$

where V is the velocity for the wheel drive, S is the wheel drivedistance from the turning, reference, S_(R) is the reference distance,and V_(R) is the vehicle velocity.
 26. The method of claim 19 furthercomprising the logic for determining a steering angle for at least onewheel drive.
 27. The method of claim 19 further comprising logic foroutputting the determined steering angle to the at least one drive. 28.A method of driving a multiple wheel drive vehicle comprising the stepsof: (a) reading an angle value associated with a steering position; (b)determining a velocity for at least one wheel drive based on the anglevalue, a vehicle reference point's velocity and location from apredetermined origin, and at least one wheel drive base dimension; and(c) outputting the determined velocity to the at least one wheel drive.29. A system for driving a multi-wheel drive vehicle comprising: (a)means for inputting at least one control signal; (b) a controller meansin circuit communication with the means for inputting a plurality ofcontrol signals; (c) at least one wheel drive means in circuitcommunication with the controller means; (d) means for determining avelocity for the at least one wheel drive means based on the at leastone control signal, a vehicle reference point's velocity and locationfrom a predetermined origin, and at least one wheel drive basedimension; and (e) output means conveying the determined velocity to theat least one wheel drive.